Muffler



MUFFLER Filed Nov. 3, 1955 2 Sheets-Sheet 1 INVENTORQ 1 4,5501% BY K x L. L. BERANEK MUFFLER Filed Nov. 3, 1955 2 Sheets-Sheet 2 INVENTOR. [E Liam 4 AT TOQNEV United States Patent MUFFLER Leo L. Beranek, Winchester, Mass, assignor to General Dynamics Corporation, San Diego, Calif., a corporation of Delaware The present invention relates to exhaust mufflers and .more particularly to an exhaust mufller designed for use with internal combustion engines.

In the operation of internal combustion engines, the burning of fuel produces a succession of pulsations or slugs of exhaust gases which travel through the associated exhaust system and thence to the atmosphere. These pulsations of gases are characterized by fluctuating pressures, high temperatures, and high velocities through the exhaust manifold, producing a series of loud explosions upon expansion of the gases into the atmosphere.

It is the primary function of a muffler to smooth out pressure peaks due to exhaust pulsations in the manifold and thereby effect a reduction in exhaust noise. The mufiler should be simple and comparatively inexpensive to construct, should impose a minimum performance penalty on the engine, and, in the case of aircraft, should be adapted to a flush or aerodynamically clean installation in the wing section aft of the usual engine nacelle. The mufiler of the present invention is such a muffler.

The present mufiier is designed to smooth out pressure peaks in the exhaust pulsations of an internal combustion engine with a minimum of back pressure for maximum engine efficiency. Further, the present mufrler is substantially rectangular in cross section and, in certain aircraft installations where the engine exhaust is directed over or under the trailing edge of the wing, this makes possible an aerodynamically clean installation. The dimensions of the rectangular mufller are adjusted so that the major cross sectional axis of the rectangle extends normal to the direction of flow of the exhaust gases without having any large portion of the muffier extend above or below the wing into the airstream. In addition, sufiicient cross sectional area is provided for the free escape of exhaust gases. The rectangular shape is preferred to a circular configuration wherein to obtain the necessary cross sectional area for effective performance, a muffler would be required which protrudes above or below the wing surfaces in an aerodynamically undesirable manner.

A series of streamlined structural members or struts are disposed across the muffler throat to permit the employment of lightweight materials without appreciable loss of structural rigidity. In addition, the rearward end of the mufller is faired or disposed to one side so that residual exhaust noise is directed away from adjoining structure such as, for example, the fuselage of anaircraft. Circumferential structural members are spaced along the length of the muffler, serving not only to stiffen the mufiier structure, but also to retain acoustical material in position within the individual acoustical chambers formed by such circumferential members. It is noted that these circumferential members, by forming a series of individual circumferential chambers, greatly reduce the transmission of flanking sound through the muflier lining.

Larger or deeper acoustical chambers are provided at the mufiler sides, while thinner acoustical chambers are provided in the upper and lower portions of the mufiler.

The chambers are acoustically interconnected with the interior of the muffler by means of a plurality of holes in the inner muffler lining, so that exhaust pressure pulsations may be admitted to the acoustical chambers for damping or attenuation. In addition, the size of the chambers are made of a depth such that in combination with the size and spacing of holes in the muffler lining, a resonance condition is produced at a frequency just below the frequency where a half wave length of sound could stand between the side faces and between the upper and lower faces of the muffler lining.

It is therefore an object of the present invention to provide an improved silencer or muffler for muffiing the sound of exhaust gases with a minimum back pressure upon the gases.

Another object of the invention is to provide a unique muffler device which is characterized by a rectangular configuration which permits aerodynamically smooth mounting in an aircraft wing, and which is faired or disposed at its rearward end to direct the flow of exhaust gases away from the fuselage of the aircraft.

It is another object of the invention to provide. a novel muffier which is adapted for smoothing out the peaks of pressure fluctuations of exhaust gases from an internal combustion engine through the provision of a plurality of perforations communicating with acoustical side chambers and upper and lower chambers, the side chambers being of greater cross sectional width than the upper and lower chambers.

Still another object of the invention is to provide a novel mufiler which incorporates a series of streamlined struts which serve to space apart and rigidify inner muffler tube forming means which are acoustically interconnected with sound damping chambers disposed about the inner mufiler tube.

An additional object of the invention resides in the provision of an improved muiiler which is of rectangular configuration and cross section, and which embodies a plurality of circumferential structural members disposed along the length of the muffler to form acoustical chambers in communication with the interior of the muffier and adapted to house acoustical lining material.

Another object of the present invention is to provide an improved muiller ;which is comparatively light in weight, simple and economical to manufacture, and adapted for rapid installation and removal.

Other objects and features of the present invention will be readily apparent to those skilled in the art from the following specification and appended drawings wherein is illustrated a preferred form of the invention, and inwhich:

Figure 1 is a perspective view of the muffler of the present invention, illustrated in a typical installation in an aircraft;

Figure 2 is a plan View of the present muflier;

Figure 3 is a view taken along IIIIII of Figure 2;

Figure 4 is a view taken along IVIV of Figure 2; and

Figure 5 is a detail elevational cross sectional view of the construction and contents of a typical acoustical chamber.

Referring to the drawings and more particularly to Figure 1, there is illustrated a preferred embodiment of the mufller of the present invention which will be described in connection with an internal combustion engine of an aircraft. It is to be understood, of course, that the particular application described is merely illustrative and the present invention is not necessarily to be limited thereto.

The exhaust muffler is generally designated by the numeral '11, and comprises an elongtaed, open-ended foraminous tubing or casing 12 of rectangular shape, an imperforate or closed rectangular outer casing or housg 3 ing 13 arranged about casing :12 to define an annular space therebetween, and spaced apart therefrom by a plurality of longitudinally spaced structural members or Z-stringers 14, which are circumferentially disposed about casing 12. Muffler .11 is illustrated in Figure 1 in conjunction .with a pair of usual engine exhausts 15 and 16, while the rearward portion of a port engine nacelle 17 and a wing 1-8 of the aircraft are illustrated in phantom outline to indicate the relative location of nacelle 17, mufiler 1'1, and wing 18. It is to be noted that the rectangular configuration of muffler 11 adapts mufller 11 to the substantially rectangular shape of naeelle 17, thereby generally following the contour of wing 18 to minimize aerodynamic resistance. As shown,one corner of muffler 11, Figure 4, may be cut back slightly for a particular application to adapt the muffler to a confined space, however, it is understood that this cut back construction does not form a part of the present invention.

The major axis of muffler 1:1 is along the span of the wing, and in this manner only a small portion of the ,main body of mulfler 11 extends above the surfaces of wing 18 so that an aerodynamically smooth installation is presented as compared with the undesirable installation which would be afforded by a circular configuration having the same cross sectional area. Further, as will'be explained in greater detail hereinafter, the rearward portion of muffler '11 is faired to the left, as viewed 'in Figure 1, so that exhaust gases and the noise associated therewith are directed away from the fuselage (not shown) which it is to'be assumed is here located to the right of muffler 11. In this manner, the present invention provides a muffler which is compact and adapted for ready securernent to the usual aircraft engine exhaust manifolds. v

'A transition assembly 19, Figure 2, serves to connect the forward portion of mufller 11 to the rearward portions of exhausts 15 and 16, and comprises a pair of cylinders or barrels which diverge rearwardly to form an integral and substantially rectangular structure, as illustrated. The rearward periphery of assembly 19 is flanged circumferentially, "as at 23, and this flanged portion23 is suitably secured, as by riveting, to the most forward stringer 14. This stringer 114 also serves to close the forward end of muffler 11 except to the inlet barrels of assembly 19.

Stringers 14 which, as stated, serve to space apart casing 12 and housing 13, are arranged circumferentially about casing 12 at spaced intervals along the length of the casing. Each encircling stringer 14 includes, as best seen in Figure 5, an upright portion '25, a forwardly extending integral flange 26, and a rearwardly extending integral flange 27, portions 25 serving together with casing 12 and housing 13 to form a plurality of acoustical chambers extending circumferentially about casing 112. Flanges 26 serve as structure to which housing 13 may be secured by rivets, screws, or the like, and flanges 27 of stringers 14 provide structure to which casing 12 may be secured by similar fastening devices. a

It is noted that each stringer 14 is formed wider at the side portions thereof so as to provide thicker or wider acoustical spaces at the sides of muffler 11, as compared with the thickness or height of the spaces at the upper I resonance condition is produced at a frequency just below the frequency where a half wave length of sound could stand between opposed faces of casing 12.

, For practical reasons, including convenience of manufacture, casing 12 and housing 13 are made of separate formed sheet metal sections which a e connected together by any suitable means such as welding or riveting, as best seen in Figure 4. Housing .13 is irnperforate, while casing 12 is provided with a large plurality of spaced openings 28 which are acoustically interconnected with the plurality of chambers formed by stringers 14, casing -12 and housing 13. The holes or perforations 28 in casing 12 have predetermined diameters and are spaced at calculated intervals to achieve the desired sound damping, the diameter and spacing of perforations 28 being arrived at by theoretical and test methods well known to those skilled in the art of sound attenuation. The size of the exhaust outlet, the structural dimensions of the mufller, the noise frequencies desired to be muffled, and the characteristics peculiar to the particular engine being muffled, are considered in determining optimum hole spacing and size.

The rearward or outlet portion of mufller 11 is desig nated by the numeral 29, and comprises a hollow fairing assembly which is formed from sheet metal into the configuration illustrated, or into any suitable configuration which tends to properly direct exhaust gases, minimize aerodynamic drag, and minimize back pressure to the engine. Fairing assembly 29 is internally stiffened by ribs or the like (not shown), and is suitably secured to the rearward end of the main body portion of mufller 11 by any suitable means such as by riveting or welding. It is to be noted that the configuration of the outlet portion 29 of mother '11 is f-aired so as to direct the flow of exhaust gases from muffler 11 away from the aircraft fuselage to thereby reduce the level of noise carried to the fuselage. 7

Each of the acoustical chambers formed between stringers 14 are filled above and below casing 12 with a suitable sound damping material or materials. In the present invention, as best illustrated in Figure 5, the upper and lower portions of each chamber formed between each pair of adjacent stringers 14 contains a layer of relatively open or diffused sound damping material such as wire cloth or mesh 31 adjacent to casing 12. In addition, positioned upon wire mesh 31 are a plurality of layers of fibrous and relatively dense gas pressure absorbing or sound attenuating material 32, such as mineral fiber or glass fiber Wool, layers 32 being positioned upon or outwardly of mesh 31. Although a single layer may be employed, it has been found that a plurality of layers, loosely bound together into a blanket by cord or the like, is a convenient construction for handling, and

also such a construction appears to more effectively attenuate exhaust pressure pulsations by virtue of the damping action provided at the interfaces of the layers of material 32. At the sides of rnuffler 11 between casing 12 and housing 13, and preferably also in the hollow portion of fairing assembly 29, the present invention utilizes bulk glass fiber wool packed to a suitable density, such as for example approximately ten pounds per cubic foot.

It is noted that mesh 31 not only serves to absorb a certain amount of the turbulent energy or pressure pulsations of the exhaust gases, particularly the lower frequency energy, but also mesh 31 serves to hold acoustic material 32 in position and prevent it from being ejected under the action of the exhaust gases. With this construction, the exhaust gases passing through muffler 11 will repeatedly act upon the many perforations 28 and wire mesh 31, and be acoustically transmitted to and damped out by acoustical material 32. Thus, the pressure peaks in the exhaust gas pulsations are leveled out, reducing the exhaust noise to a more or less uniform and acceptable level.

Throughout the construction of the present muffler lightweight materials have been employed wherever possible, and in the comparatively wide transverse span that is present in muffler 11, structural rigidity is preserved by the provision of a plurality of strut means 34 which are streamlined to minimize back pressure. Strutsv 34 are provided at spaced intervals along the length of mufiler 11 between the upper and lower surfaces of casing 12, and each strut 34 is provided with a suitable vertically disposed opening, through which a bolt 35, Figure 2, is disposed to secure strut 34 in position. Each bolt 35 passes through casing 12 and is secured at each end, as by a nut or the like, to an I-shaped member 36 at each end. The central web of each member 36 is enlarged, as illustrated, to provide for suitable disposition of bolt 35 therethrough, and the legs or flanges of member 36 are suitably riveted to the upright portions 25 of the pair of adjacent stringers 14. Both tensile and compressive loads may be borne by struts 34 so that structural rigidity of mufiler 11 is afforded.

It is thus evident that the mufller 11 of the present invention provides an improved silencer for muflling the sound of the exhaust gases with a minimum back pressure upon the gases, and which is characterized by a novel rectangular configuration in addition to various other unique features as above described.

While certain preferred embodiments of the invention have been specifically disclosed, it is understood that the invention is not limited thereto as many variations will be readily apparent to those skilled in the art and the invention is to be given its broadest possible interpretation within the terms of the following claim.

I claim:

An exhaust mufller in combination with an engine nacelle of an aircraft wing, said mufller comprising a longitudinally extending housing having a rectangular cross-sectional configuration, said housing being mounted to said nacelle with the major transverse axis of said housing lying along the span of said wing whereby said housing forms an aerodynamically smooth extension of said nacelle, an open-ended casing disposed within and spaced from said housing and having walls defining a rectangular opening therethrough for the passage of exhaust gases, said casing having a plurality of perforations therethrough, a plurality of structural members disposed between said casing and said housing for connecting said casing to said housing and arranged peripherally about said casing at spaced longitudinal intervals to form a plurality of individual annular sound attenuation chambers, wire fabric material arranged adjacent said casing, a plurality of layers of fibrous material arranged adjacent said wire fabric material and within said annular chambers, and streamlined struts connected across said casing at spaced longitudinal intervals, the spacing between said casing and housing and the size and spacing of said perforations all being predetermined such that a resonance condition exists at a frequency just below the frequency at which a half wave length could stand between opposed Walls of said casing, whereby audible noise in the aircraft fuselage due to the passage of said gases is reduced.

References Cited in the file of this patent UNITED STATES PATENTS 1,522,032 Scarff Jan. 6, 1925 1,978,678 Kurth Oct. 30, 1934 2,043,731 Bourne June 9, 1936 2,146,028 Reynolds Feb. 7, 1939 2,249,271 Davis July 15, 1941 2,420,964 Miller et a1 May 20, .1947 2,583,366 Engels Jan. 22, 1952 2,674,336 Lemmerman Apr. 6, 1954 

